Processes for producing β-halogeno-α-amino-carboxylic acids and phenylcysteine derivatives and intermediates thereof

ABSTRACT

An industrially advantageous method of producing beta-halogeno-alpha-aminocarboxylic acids is provided. Methods are also provided of producing optically active N-protected-S-phenylcysteines having high optical purity and of intermediates thereof, respectively, in which the above production method is utilized.A method of producing beta-halogeno-alpha-aminocarboxylic acids or salts thereof is disclosed which comprises halogenating the hydroxyl group of a beta-hydroxy-alpha-aminocarboxylic acid (in which the basicity of the amino group in alpha-position is not masked by the presence of a substituent on said amino group) or a salt thereof with an acid with a halogenating agent. A method of producing optically active N-protected-S-phenylcysteines represented by the general formula (3) or salts thereof is further disclosed which comprises applying the above production method to optically active serine or a salt thereof and then carrying out treatment with an amino-protecting agent and reaction with thiophenol under a basic condition.

TECHNICAL FIELD

The present invention relates to a method of producing aβ-halogeno-α-aminocarboxylic acid or a salt thereof, which is useful asa raw material for the production of medicinals, among others. Theinvention also relates to a method of producing an optically activeN-protected S-phenyl-L-cysteine or a salt thereof, which is useful as anintermediate of medicinals, in particular anti-AIDS drugs, and to amethod of producing an intermediate thereof.

BACKGROUND ART

The following methods, among others, are known for producingβ-halogeno-α-aminocarboxylic acids:

(1) The method which comprises derivatizing aβ-hydroxy-α-aminocarboxylic acid into the correspondingβ-hydroxy-α-aminocarboxylic acid ester, then halogenating the hydroxylgroup thereof with a phosphorus halide to give the correspondingβ-halogeno-α-aminocarboxylic acid ester, and hydrolyzing the ester groupusing a hydrohalogenic acid to give the objectiveβ-halogeno-α-aminocarboxylic acid. Specifically, serine is derivatizedinto serine methyl ester hydrochloride, the ester salt is then treatedwith phosphorus pentachloride to give α-amino-β-chloropropionic acidmethyl ester hydrochloride, which is further hydrolyzed withhydrochloric acid. The resulting α-amino-β-chloropropionic acidhydrochloride is isolated by concentrating the reaction mixture todryness, followed by crystallization of the residue from a mixture of1-propanol and hydrochloric acid [e.g. CHIRALITY, 8:197-200 (1996)]; and

(2) The method which comprises treating β-phenylserine monohydrate withthionyl chloride and then with concentrated hydrochloric acid to giveβ-chloro-β-phenylalanine [Gazzetta Chimica Italiana, 119 (1989) p. 215].

However, in the above method (1), the halogenation of the hydroxyl groupin β position usually involves three reaction steps, namely protectionof the carboxyl group, halogenation of the hydroxyl group in β positionand deprotection of the carboxyl group. In this case, many difficultiesare encountered, for example the multiplicity of steps required,procedural complexity and low yields.

In the above method (2), such difficulties arise as the use of thionylchloride in large amounts for the same to serve also as a solvent andthe resulting complicatedness of procedure. As a result ofinvestigations made by the present inventors, it was further found thatthe method is hardly applicable to the chlorination of serine, threonineor the like.

Thus, no efficient technology has been established for producingβ-halogeno-α-aminocarboxylic acids on a commercial scale.

On the other hand, such methods of producing optically activeS-phenylcysteine derivatives as mentioned below are known in the art:

<Derivatization from Serine>

1) The method comprising reacting serine with thiophenol in the presenceof tryptophan synthase (EP 754759);

2) The method which involves lactonization of a serine derivative withan azodicarboxylic acid ester [J. Am. Chem. Soc., 1985, vol. 107, p.7105; Synth. Commun., 1995, vol. 25 (16), p. 2475];

3) The method comprising converting the hydroxyl group of an N-protectedserine ester derivative to a sulfonyloxy group and substituting athiophenyl group therefor [Tetrahedron Lett., 1987, vol. 28, p. 6069;ibid., 1993, vol. 34, p. 6607; EP 604185 A1];

<Derivatization from Starting Compounds other than Serine>

4) The method comprising reacting cysteine with a phenyldiazonium saltin the presence of a copper salt [J. Org. Chem., 1958, vol. 23, p.1251];

5) The method comprising derivatizing from an aziridinecarboxylic acidderivative in the presence of boron trifluoride-ethyl ether complex[Bull. Chem. Soc. Jpn, 1983, vol. 56, p. 520];

6) The method comprising reacting cysteine with iodobenzene in thepresence of a copper salt [Aust. J. Chem., 1985, vol. 38, p. 899]; and

7) The method comprising reacting dehydroalanine with a chiral nickelcomplex [Tetrahedron, 1988, vol. 44, p. 5507].

Since optically active serine, in particular L-serine, is a readilyavailable compound, a practical method would be provided if the startingmaterial L-serine could be converted efficiently to an optically activeS-phenylcysteine derivative. However, the method 1), in which aparticular enzyme is utilized, and the method 2), in which a lactonederivative is used as an intermediate, have problems from the viewpointof operability, productivity, safety in reagents handling, and economy,among others. The method 3), in which the hydroxyl group of anN-protected serine ester derivative is converted to a sulfonyloxy groupand the resulting product is then subjected to substitution reactionusing the sodium salt of a thiol in N,N-dimethylformamide, is alsodisadvantageous in that because it involves the use of a reagentrelatively difficult to handle, for example sodium hydride or potassiumhydride, as a base, it does not always give the desired N-protectedS-phenylcysteine ester in high yield and, in particular, the opticalpurity is decreased, as revealed by a study made by the presentinventors.

On the other hand, the methods 4) through 7), which comprisederivatization from other starting compounds than serine, cannot be saidto be industrially advantageous, either, since, for example, the wastetreatment is troublesome, materials requiring caution in handling orexpensive materials are used and the yield and productivity are low.

In view of the above state of the art, the primary object of the presentinvention is to provide a method of producingβ-halogeno-α-aminocarboxylic acids in an industrially advantageousmanner and a method of producing optically active S-phenylcysteinederivatives from optically active serine, which is readily availablecommercially, in an industrially advantageous manner.

SUMMARY OF THE INVENTION

As a result of their intensive investigations made in an attempt todevelop an industrially advantageous method of producingβ-halogeno-α-aminocarboxylic acids, the present inventors havesurprisingly found an industrially advantageous production methodaccording to which β-halogeno-α-aminocarboxylic acids can be synthesizedin an efficient manner by treating aβ-hydroxy-α-aminocarboxylic acid ora salt thereof with an acid with a halogenating agent.

On the other hand, in efficiently producing optically activeS-phenylcysteine derivatives from optically active serine, namely L- orD-serine, the point is how to prevent the optical purity from decreasingin thiophenylating the activated compound derived from optically activeserine by converting its hydroxyl group to a leaving group. The presentinventors thought that there would be the possibility of attaining theabove object in an industrially advantageous manner while preventingracemization if an adequately activated carboxylic acid derivative couldbe synthesized from optically active serine by activating the hydroxylgroup thereof in the form of a leaving group and if the thiophenylationcould be realized efficiently. Based on this way of thinking, they madeintensive investigations and, as a result, found that optically activeβ-chloroalanine can be synthesized in an efficient manner when the abovemethod of producing β-halogeno-α-aminocarboxylic acids is utilized.There are no prior art findings teaching or suggesting that opticallyactive β-chloroalanine can be produced by directly chlorinatingoptically active serine or a salt thereof. The relevant method ofproduction is thus novel.

In addition, it was found that the optically active β-chloroalanineobtained in the above manner can be converted to an optically activeN-protected-β-chloroalanine by treatment with an amino-protecting agentand that said compound can be converted to an optically activeN-protected-S-phenylcysteine by reacting with thiophenol under a basiccondition. Based on these and other findings, the present invention hasnow been completed. Particularly when the above three-step process isused, optically active N-protected-S-phenylcysteine derivatives can beproduced in an industrially advantageous manner without any substantialreduction in the optical purity of the starting material, namelyoptically active L- or D-serine.

Thus, the present invention relates to a method of producing aβ-halogeno-α-aminocarboxylic acid or a salt thereof

which comprises halogenating the hydroxyl group of aβ-hydroxy-α-aminocarboxylic acid (in which the basicity of the aminogroup in a-position is not masked by the presence of a substituent onsaid amino group) or a salt thereof with an acid by treating the samewith a halogenating agent.

The present invention also relates to a method of producing an opticallyactive N-protected-β-chloroalanine of the general formula (2) or a saltthereof according to the above method of production:

wherein R¹ represents an amino-protecting group and R⁰ represents ahydrogen atom or, taken together with R¹, an amino-protecting group,

namely by preparing an optically active β-chloroalanine of the formula(1) or a salt thereof:

from an optically active serine or a salt thereof with an acid, and thentreating the same with an amino-protecting agent.

The present invention further provides a method of producing anoptically active N-protected-S-phenylcysteine of the general formula (3)or a salt thereof:

wherein R¹ represents an amino-protecting group and R⁰ represents ahydrogen atom or, taken together with R¹, an amino-protecting group,

which comprises preparing an optically activeN-protected-β-chloroalanine or a salt thereof according to theproduction method mentioned above and then reacting the same withthiophenol under a basic condition.

In the following, the invention is described in detail.

DETAILED DISCLOSURE OF THE INVENTION

The β-hydroxy-α-aminocarboxylic acid to be used in the practice of theinvention is not particularly restricted but, basically, is one whoseamino group retains its basicity without being masked by the presence ofa substituent thereon, for example an acyl type amino-protecting group.The basic skeleton of the above β-hydroxy-α-aminocarboxylic acid isα-amino-β-hydroxypropionic acid (also called serine), and one, two orthree of the three hydrogen atoms on the carbon chain other than thoseof the amino, hydroxyl and carboxyl groups of the basic skeleton may besubstituted with another group or other groups unless the halogenationreaction is adversely affected. Further, one or two of the hydrogenatoms of the above amino group may be substituted with a substituent orsubstituents (e.g. alkyl, aralkyl, aryl, etc.) unless the halogenationreaction is adversely affected and unless the basicity of the aminogroup is jeopardized.

As typical examples of the β-hydroxy-α-aminocarboxylic acid, there maybe mentioned, among others, serine, threonine, allothreonine,β-phenylserine and the like. The salt of the β-hydroxy-α-aminocarboxylicacid with an acid is not particularly restricted, either, but includes,among others, such salts as serine hydrochloride, threoninehydrochloride, allothreonine hydrochloride and β-phenylserinehydrochloride. The above salt may be prepared and isolated in advance,or may be prepared in the reaction vessel or formed during reaction.When these β-hydroxy-α-aminocarboxylic acids are used, the products areβ-halogeno-α-aminopropionic acids (i.e. β-haloalanines), β-halogeno-α-aminobutyric acids, β-halogeno-β-phenyl-α-aminopropionic acids (i.e.β-halophenylalanines), etc. It is a matter of course that the aboveβ-hydroxy-α-aminocarboxylic acids may be used in an optically activeform.

The halogenating agent to be used in the practice of the inventionincludes, among others, thionyl halides and phosphorus halides,specifically thionyl chloride, thionyl bromide, phosphoruspentachloride, phosphorus trichloride, phosphorus oxychloride,phosphorus tribromide, etc. From the viewpoint of reaction yield andease of handling, however, thionyl halides are preferred, in particularthionyl chloride is most preferred. The above halogenating agent is usedin an amount of, for example 1 to 10 moles, preferably 1 to 4 moles,more preferably 1 to 2 moles, per mole of the substrateβ-hydroxy-α-aminocarboxylic acid or a salt thereof with an acid.Basically, the above amount is the number of moles of the basic skeletalunit of the βhydroxy-α-aminocarboxylic acid and, in cases where aplurality of such basic skeletal units as mentioned above are containedin each molecule or where the another or other substituents consume thehalogenating agent or a group consuming said agent is contained, forinstance, it is considered necessary to increase the amount of thehalogenation agent by the corresponding equivalent amount.

The treatment with the halogenating agent in the production method ofthe present invention is preferably carried out in a solvent. Preferredas the solvent in that case are, for example, 1,2-dimethoxyethane,1,4-dioxane, tetrahydrofuran, diethylene glycol dimethyl ether,triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether,polyethylene glycol dimethyl ether, tert-butyl methyl ether, dibutylether, diethyl ether and like ether solvents; acetonitrile, methylenechloride, ethyl acetate and other aprotic solvents. These maybe usedsingly or two or more of them may be used combinedly. Among them, ethersolvents are preferred and, in particular, ether solvents miscible withwater, such as 1,2-dimethoxyethane, 1,4-dioxane, tetrahydrofuran,diethylene glycol dimethyl ether, triethylene glycol dimethyl ether,tetraethylene glycol dimethyl ether and polyethylene glycol dimethylether, are more preferred. It is of course possible to use anothersolvent or other solvents within limits within which no adverse effectis produced.

The treatment with the above halogenating agent can be carried out inthe presence of an amine or a salt thereof. The amine or the saltthereof is not particularly restricted but includes, for example,triethylamine, trimethylamine, diisopropylethylamine,tetramethylethylenediamine, pyridine, dimethylaminopyridine, imidazole,triethylamine hydrochloride, trimethylamine hydrochloride,diisopropylethylamine hydrochloride and the like. Among them, tertiaryamines such as trimethylamine and triethylamine or salts thereof arepreferred. More preferred is triethylamine or its hydrochloride.

The above amine or its salt is added preferably in an amount of 0.1 to30 mole percent, more preferably 1 to 10 mole percent, based on thesubstrate β-hydroxy-α-aminocarboxylic acid or a salt thereof.

In a mode of practice of the present invention which is more preferredin attempting to attain a higher reaction yield, the treatment with theabove halogenating agent, preferably thionyl chloride, is carried out inthe presence of a hydrogen halide, preferably hydrogen chloride (gas).The hydrogen halide is used in an amount of, for example, not less thanabout 1 molar equivalent, preferably an amount exceeding 2.0 molarequivalents, more preferably an amount of not less than about 3 molarequivalents, based on the β-hydroxy-α-aminocarboxylic acid. Generally,by using the hydrogen halide in an amount of about 3 to 10 molarequivalents, it is possible to carry out the above treatment verysmoothly. Like the case mentioned above, it is fundamentally understoodthat the amount mentioned above corresponds to the number of molarequivalents per basic skeletal unit of the β-hydroxy-α-aminocarboxylicacid (the hydrohalogenic acid salt of a β-hydroxy-α-aminocarboxylic acidcorresponds to the presence of 1.0 molar equivalent of the correspondinghydrogen halide relative to the β-hydroxy-α-aminocarboxylic acid). Theconcentration of the hydrogen halide in the reaction mixture is, forexample, not less than about 1 mole, preferably not less than about 2moles, more preferably not less than about 3 moles, per liter ofsolvent. The above treatment can be carried out smoothly at a hydrogenhalide concentration not higher than the saturated concentration in thereaction system. The above treatment may be carried out in the presenceof an amine or a salt thereof.

Referring specifically to a simple reaction procedure taken as anexample, a suspension composed of a β-hydroxy-α-aminocarboxylic acid(e.g. L-serine) and 1,4-dioxane, for instance, is almost or completelysaturated with hydrogen chloride gas, thionyl chloride is then addedand, after completion of the addition, the mixture is moderaly orvigorously stirred preferably at room temperature to 100° C., morepreferably at 40 to 80° C., preferably for 0.5 to 30 hours, morepreferably for 1 to 20 hours, to give the correspondingβ-chloro-α-aminocarboxylic acid [e.g. L-α-amino-β-chloropropionic acid(also called β-chloro-L-alanine)].

The β-halogeno-α-aminocarboxylic acid obtained by the above halogenationmay be isolated prior to the use in the next step or may be used withoutisolation.

The above β-halogeno-α-aminocarboxylic acid may be isolated, forexample, by such a technique as column chromatography commonly used inisolating amino acids. Said acid can be isolated in a simple andefficient manner by the method mentioned below, however.

For isolating the above β-halogeno-α-aminocarboxylic acid in the form ofa hydrohalogenic acid salt, for example hydrochloride, after completionof the reaction, during which the precipitation of the desired productproceeds (namely reaction/crystallization proceeds) with the progress ofthe treatment with the above halogenating agent, the reaction mixture issubjected, either as such or after concentration, to conventionaltreatment for solid-liquid separation, such as filtration orcentrifugation, whereby the desired product can be recovered in a verysimple manner and in high yields. In the step of isolation, it is ofcourse possible to reduce the content of or remove those relatively lowboiling components remaining in the reaction mixture after thehalogenation reaction, such as sulfur dioxide, the excess hydrogenhalide (e.g. hydrogen chloride) and the unreacted halogenating agent(e.g. thionyl halide), in advance, according to need. By concentratingthe reaction mixture, it is also possible to recover the reactionsolvent.

For isolating the above β-halogeno-α-aminocarboxylic acid in the freeform, the acid coexisting in the reaction mixture after the halogenationreaction is converted to a salt, preferably a salt soluble in an organicsolvent and water (e.g. lithium halide such as lithium chloride) using abase, preferably a basic lithium compound such as lithium hydroxide orlithium carbonate, for instance, and the aboveβ-halogeno-α-aminocarboxylic acid is caused to crystallize out from anorganic solvent, water or a medium composed of an organic solvent andwater while causing dissolution of the above resulting salt in suchmedium. The subsequent separation using a conventional solid-liquidseparation procedure, such as filtration or centrifugation, gives thedesired product in a simple and convenient manner. Since, generally, theconversion of acids to salts is preferably carried out in the presenceof water, it is desirable to attempt to reduce the solubility of theβ-halogeno-α-aminocarboxylic acid, which is a water-soluble compound,or, in other words, increase the precipitate amount, by using awater-miscible organic solvent as said organic solvent.

The above water-miscible organic solvent specifically includes, but isnot limited to, 1,2-dimethoxyethane, 1,4-dioxane, tetrahydrofuran,diethylene glycol dimethyl ether, triethylene glycol dimethyl ether,tetraethylene glycol dimethyl ether, polyethylene glycol dimethyl ether,acetonitrile, methanol, ethanol, n-propanol, isopropanol, tert-butanoland acetone, among others. Among these, acetone, in particular, ispreferred from the viewpoint of increased precipitation of water-solubleβ-halogeno-α-aminocarboxylic acids, production of crystals having goodcharacteristics, ease of handling and inexpensiveness, among others.

Since the above β-halogeno-α-aminocarboxylic acid has a high solubilityin water, it is desirable, for attaining increased precipitation, toreduce the amount of water, use the above water-miscible organic solventin a volume ratio of not less than 1 relative to water and maintain thefinal cooling temperature at a low level, preferably not higher than 10°C., more preferably not higher than 0° C. The solubility of the aboveβ-halogeno-α-aminocarboxylic acid tends to increase in the presence oflithium chloride or the like, hence it is effective to use acetonecombinedly so that the precipitation may be maximized.

In the step of adding a basic lithium compound for converting thecoexisting acid to the salt form for causing precipitation of the aboveβ-halogeno-α-aminocarboxylic acid, the reaction mixture is preferablyadjusted to weak acidity to neutrality, specifically to the vicinity ofthe isoelectric point of the β-halogeno-α-aminocarboxylic acid. When theβ-halogeno-α-aminocarboxylic acid is an α-amino-β-halopropionic acid orα-amino-β-halobutyric acid, the pH is preferably adjusted to about 4 to7.

Specifically, in a simple procedure, taken as an example, for isolatingthe above β-halogeno-α-aminocarboxylic acid in its free form, thoserelatively low boiling components remaining in the reaction mixtureafter the halogenation reaction, such as sulfur dioxide, excess hydrogenhalide (e.g. hydrogen chloride) and unreacted halogenating agent (e.g.thionyl halide), are preferably reduced in amount or removed in advance,the pH is then adjusted at a low temperature using a basic lithiumcompound such as lithium hydroxide or lithium carbonate, preferablylithium hydroxide, a small amount (preferably minimum amount) of water,and the resulting precipitate, i.e. β-halogeno-α-aminocarboxylic acid,is collected using a medium mainly comprising a water miscible organicsolvent used as the halogenation reaction solvent, preferably awater-miscible ether solvent. Alternatively, after reducing or removingin advance those relatively low boiling components remaining in thereaction mixture after the halogenation reaction, such as sulfurdioxide, the excess hydrogen halide (e.g. hydrogen chloride) and theunreacted halogenating agent (e.g. thionyl halide), the reaction solventis replaced with a small amount (preferably minimum amount) of water ata low temperature and, if necessary after treatment with an adsorbentsuch as activated carbon and/or separation of the insoluble matter byfiltration for the purpose of removing impurities and/or decoloration,the pH is adjusted using a basic lithium compound such as lithiumhydroxide or lithium carbonate, preferably lithium hydroxide and a smallamount (preferably minimum amount) of water, the precipitation of theβ-halogeno-α-aminocarboxylic acid is fully caused by combinedly usingthe above water-miscible organic solvent, preferably acetone; said acidcan then be recovered.

In cases where the above β-halogeno-α-aminocarboxylic acid is submittedto the next step without isolation, those relatively low boilingcomponents remaining in the reaction mixture after the halogenationreaction, such as sulfur dioxide, the excess hydrogen halide (e.g.hydrogen chloride) and the unreacted halogenating agent (e.g. thionylhalide), are reduced or removed beforehand, and the reaction solvent isreplaced with water at a low temperature, for instance, and, ifnecessary the pH is adjusted with a base such as sodium hydroxide orlithium hydroxide and, further, if necessary treatment with an adsorbentsuch as activated carbon and/or separation of the insoluble matter byfiltration is conducted for the purpose of removing impurities and/ordecoloration, whereafter the above β-halogeno-α-aminocarboxylic acid canbe used in the form of an aqueous solution.

A preferred method of purifying and isolating the aboveβ-halogeno-α-aminocarboxylic acid is now described. This is a method ofpurifying and isolating the β-halogeno-α-aminocarboxylic acid in itsfree form. In the method (1) mentioned below, theβ-halogeno-α-aminocarboxylic acid can be used and, in the method (2)mentioned below, the β-halogeno-α-aminocarboxylic acid or a salt thereofcan be used, and the salt of the β-halogeno-α-aminocarboxylic acid ispreferably a hydrohalogenic acid salt such as hydrochloride. It is ofcourse possible to use an optically active form of the aboveβ-halogeno-α-aminocarboxylic acid.

(1) Using water as a good solvent and a water-miscible organic solventas a poor solvent, the β-halogeno-α-aminocarboxylic acid is caused tocrystallize out. Preferably, the β-halogeno-α-aminocarboxylic acid iscaused to crystallize out from an aqueous solution thereof in thepresence of a water-miscible organic solvent. If necessary, treatmentwith an adsorbent such as activated carbon and/or filtration of theinsoluble matter may be combined for the purpose of removing impuritiesand/or decoloration.

(2) Treatment of the aqueous solution containing theβ-halogeno-α-aminocarboxylic acid and hydrogen halide with a basiclithium compound, such as lithium hydroxide or lithium carbonate, forconverting the (hydrohalogenic) acid to the salt is combined withprecipitation of the β-halogeno-α-aminocarboxylic acid in its free formusing water as a good solvent and a water-miscible organic solvent as apoor solvent. Basically, the above-mentioned technique for isolating theβ-halogeno-α-aminocarboxylic acid in its free form from the halogenationreaction mixture can be utilized. Preferably, theβ-halogeno-α-aminocarboxylic acid or a salt thereof (preferably ahydrohalogenic acid salt such as hydrochloride) is first caused tocoexist with, preferably dissolved in, an aqueous solution of ahydrohalogenic acid, such as hydrochloric acid, or water. The pH isadjusted generally to 3 or below, preferably to 2 or below, and theamount of water required for fluidization, preferably dissolution ispreferably minimized. Then, if necessary, treatment with an adsorbentsuch as activated carbon and/or insoluble matter separation byfiltration is carried out for the purpose of removing impurities and/ordecoloration. While adjusting the pH with a basic lithium compound suchas lithium hydroxide or lithium carbonate, the hydrohalogenic acid isconverted to a salt (a lithium halide such as lithium chloride) solublein the organic solvent and water, and the β-halogeno-α-aminocarboxylicacid is caused to precipitate using the water-miscible organic solventas a poor solvent while the above salt is caused to remain dissolvedwithout precipitation. Thereafter, the acid is recovered by aconventional solid-liquid separation procedure, such as filtration orcentrifugation. Alternatively, the β-halogeno-α-aminocarboxylic acid ora salt thereof (preferably a hydrohalogenic acid salt thereof, such ashydrochloride) is dissolved in a medium comprising water or an aqueoussolution of a hydrohalogenic acid, such as hydrochloric acid, and anorganic solvent miscible with water. The pH after dissolution isadjusted generally to 3 or below, preferably to 2 or below. Then, ifnecessary, treatment with an adsorbent such as activated carbon and/orinsoluble matter separation by filtration is carried out for the purposeof removing impurities and/or decoloration. Theβ-halogeno-α-aminocarboxylic acid is caused to precipitate by adjustingthe pH (converting the hydrohalogenic acid, if present, to the form of asalt) using a basic lithium compound such as lithium hydroxide orlithium carbonate while the above salt formed (lithium halide such aslithium chloride) is caused to remain dissolved without precipitation.Thereafter, the desired acid is recovered by a conventional solid-liquidseparation procedure such as filtration or centrifugation.

The water-miscible organic solvent to be used in the above methods (1)and (2) specifically includes, but is not limited to,1,2-dimethoxyethane, 1,4-dioxane, tetrahydrofuran, diethylene glycoldimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycoldimethyl ether, polyethylene glycol dimethyl ether, acetonitrile,methanol, ethanol, n-propanol, isopropanol, tert-butanol and acetone,among others. Among these, acetone, in particular, is preferred from theviewpoint of increased precipitation of the β-halogeno-α-aminocarboxylicacid, which is a water-soluble compound, obtaining crystals with goodcharacteristics, ease of handling and inexpensiveness, among others.

Since the β-halogeno-α-aminocarboxylic acid has a high solubility inwater, it is desirable, for attaining increased precipitation, to reducethe amount of water, use the above water-miscible organic solvent in avolume ratio of not less than 1 relative to water and maintain the finalcooling temperature at a low level, preferably not higher than 10° C.,more preferably not higher than 0° C. The solubility of the aboveβ-halogeno-α-aminocarboxylic acid tends to increase in the presence oflithium chloride or the like, hence it is effective to use acetonecombinedly so that the precipitation may be maximized.

In the step of crystallization or precipitation of the aboveβ-halogeno-α-aminocarboxylic acid, the pH is adjusted to weak acidity toneutrality, specifically to the vicinity of the isoelectric point of theβ-halogeno-α-aminocarboxylic acid. When the β-halogeno-α-aminocarboxylicacid is an α-amino-β-halopropionic acid or α-amino-β-halobutyric acid,the pH is preferably adjusted to about 4 to 7.

Most preferred as the above hydrohalogenic acid is hydrogen chloride(hydrochloric acid) and, as the above basic lithium compound, lithiumhydroxide or lithium carbonate, in particular lithium hydroxide, ispreferred.

Since the above β-halogeno-α-aminocarboxylic acid is not always stable,care is preferably taken in contacting the same with a base so as toeffect contacting thereof with water or an aqueous medium approximatelyunder acidic or neutral conditions, for instance. Generally, the acid ishandled preferably under acidic to neutral conditions, for example at apH of not higher than 7, and at low temperatures.

According to the method of the present invention,β-halogeno-α-aminocarboxylic acids can efficiently be synthesized fromβ-hydroxy-α-aminocarboxylic acids in one reaction step, and high qualityβ-halogeno-α-aminocarboxylic acids or salts thereof can be isolated inhigh yields. Further, when the above reaction is carried out using theβ-hydroxy-α-aminocarboxylic acid in an optically active form, thecorresponding optically active β-halogeno-α-aminocarboxylic acid havingthe same configuration as that of the substrate can be obtained whilethe optical purity of the starting material is substantially maintainedwithout accompanying substantial racemization.

For converting the optically active β-chloroalanine obtained from anoptically active serine or a salt thereof according to the aboveproduction method to an optically active N-protected-S-phenylcysteine,two methods are conceivable, one comprising treatment with anamino-protecting agent, followed by thiophenylation and the othercomprising treatment with an amino-protecting agent followingthiophenylation. However, studies made by the present inventors revealedthat the method comprising treatment with an amino-protecting agentfollowed by thiophenylation is preferred from the viewpoint of yield andoperability. The method comprising treatment with an amino-protectingagent following thiophenylation cannot give satisfactory yields sincethe optically active β-chloroalanine is unstable particularly underthiophenylation conditions.

The method of the present invention for producing the optically activeN-protected-β-chloroalanines of the general formula (2) given above orsalts thereof comprises producing an optically active β-chloroalanine ora salt thereof by treating an optically active serine or a salt of anoptically active serine with an acid with a chlorinating agent and thentreating that product with an amino-protecting agent. In this productionmethod, the reaction for obtaining the optically active β-chloroalanineor a salt thereof can be carried out in the same manner as mentionedabove.

In the above general formula (2), R¹ represents an amino-protectinggroup. As the amino-protecting group, there may be mentioned thosedescribed in Theodora W. Green: Protective Groups in Organic Synthesis,2nd edition, John Wiley & Sons, published 1990, such asbenzyloxycarbonyl, ethoxycarbonyl, methoxycarbonyl, tert-butoxycarbonyl,acetyl, tosyl, benzoyl, phthaloyl and the like. The range of choice alsoincludes such protective groups as (3S)-tetrahydrofuranyloxycarbonyl,3-hydroxy-2-methylbenzoyl whose hydroxyl group may optionally beprotected, and the like. However, benzyloxycarbonyl is preferred amongothers.

In the above general formula (2), R⁰ generally represents a hydrogenatom but may also represent such an amino-protecting group as phthaloyltogether with R¹.

The above amino-protecting agent corresponds to the aboveamino-protecting group and includes conventional amino-protecting agentswithout any particular restriction. Thus, mention maybe made of, forexample, benzyl chloroformate, ethyl chloroformate, methylchloroformate, di-tert-butyl dicarbonate, benzoyl chloride, acetylchloride, p-toluenesulfonyl chloride, phthalic anhydride, andN-carboethoxyphthalimide. The range of choice further includes(3S)-tetrahydrofuranyl chloroformate, 3-hydroxy-2-methylbenzoyl chloridewhose hydroxyl group may optionally be protected, and the like. Amongthem, benzyl chloroformate is preferred.

While the treatment with the above amino-protecting agent may be carriedout using an optically active β-chloroalanine isolated, it is preferredthat the amino group protection be effected by treating, with the aboveamino-protecting agent, an aqueous medium containing an optically activeβ-chloroalanine as obtained in the manner mentioned above. In eithercase, abase is used and the base to be used is, for example, sodiumhydroxide or potassium carbonate. The above treatment with anamino-protecting agent may be carried out in any medium comprising waterand/or an organic solvent.

The solvent to be used in that case is not particularly restricted butmay be, for example, 1,2-dimethoxyethane, 1,4-dioxane, tetrahydrofuran,diethylene glycol dimethyl ether, triethylene glycol dimethyl ether,tetraethylene glycol dimethyl ether, polyethylene glycol dimethyl ether,tert-butyl methyl ether, dibutyl ether, diethyl ether or a like otherether solvent; acetonitrile, methylene chloride, ethyl acetate, acetone,toluene or a like other aprotic solvent.

Taking carbobenzyloxylation as an example, the method for the aboveamino group protection is now specifically described. To an aqueousmedium containing an optically active β-chloroalanine, for instance,there is added benzyl chloroformate in an amount of 1 to 2 molarequivalents, preferably about 1.0 molar equivalent, relative to thesubstrate, at a temperature at which the solvent will not freeze, up to30° C., more preferably at a temperature not higher than 5° C., whilemaintaining the pH at 8 to 13, preferably 9 to 12, more preferably 9 to10, by adding a base, such as sodium hydroxide or potassium carbonate,and the resulting mixture is stirred at a temperature at which thesolvent will not freeze, up to 30° C., more preferably at a temperaturenot higher than 5° C., preferably for 1 to 30 hours. If necessary, thereaction mixture may be washed with an organic solvent immiscible withwater or with an aqueous medium, for example toluene, for the purpose ofremoving the unreacted portion of benzyl chloroformate and the byproductbenzyl alcohol.

The optically active N-protected-β-chloroalanine produced in the abovemanner can be isolated, for example by a conventional extractionprocedure followed by column chromatography.

The method of the present invention for producing optically activeN-protected-S-phenylcysteines of the above general formula (3) or saltsthereof comprises treating an optically active serine or a salt of anoptically active serine with an acid with a chlorinating agent, thentreating the thus-obtained optically active β-chloroalanine with anamino-protecting agent, and further reacting the resulting opticallyactive N-protected-β-chloroalanine or a salt thereof with thiophenolunder a basic condition. In the above general formula (3), R⁰ and R¹ arethe same as the R⁰ and R¹ specifically mentioned above. In thisproduction method, the reactions for the production of the opticallyactive N-protected-β-chloroalanine or a salt thereof can be carried outin the same manner as mentioned above.

The thiophenylation of the above optically activeN-protected-β-chloroalanine can be carried out using an optically activeN-protected-β-chloroalanine isolated in the manner mentioned above. Itis also possible to adjust the pH of the reaction mixture afteramino-protecting agent treatment, add thiophenol directly thereto andeffecting the reaction in that reaction mixture.

The above step of reacting the optically activeN-protected-β-chloroalanine with thiophenol can be conducted in waterand/or an organic solvent under a basic condition. The organic solventis not particularly restricted but includes, for example, ether solventssuch as 1,2-dimethoxyethane, 1,4-dioxane, tetrahydrofuran, diethyleneglycol dimethyl ether, triethylene glycol dimethyl ether, tetraethyleneglycol dimethyl ether, polyethylene glycol dimethyl ether, tert-butylmethyl ether, dibutyl ether and diethyl ether; and other aproticsolvents such as acetonitrile, methylene chloride, ethyl acetate,acetone and toluene, among others.

The above thiophenol is used generally in an amount of 1 to 5 molarequivalents, preferably 1 to 3 molar equivalents, more preferably about1.5 molar equivalents, relative to the optically activeN-protected-β-chloroalanine.

For effecting the above thiophenylation under a basic condition, aninorganic base or the like is preferably added as a base. The inorganicbase is not particularly restricted but may be, for example, sodiumhydrogen carbonate, potassium hydrogen carbonate, sodium carbonate,potassium carbonate or sodium hydroxide. An alkaline pH buffering agentmay also be used.

The amount of the above base to be used may vary depending on thespecies thereof. In the case of sodium hydroxide or sodium carbonate,for instance, it is used in an amount of 1 to 5 molar equivalents,preferably 1 to 3 molar equivalents, relative to the optically activeN-protected-β-chloroalanine. The pH of the reaction mixture ispreferably about 9 to 11. Under strongly alkaline conditions, the yieldtends to decrease due to side reactions. After completion of thereaction, the product can be isolated, for example by acidifying thereaction mixture with hydrochloric acid, sulfuric acid or the like,extracting the mixture with an organic solvent such as ethyl acetate,concentrating the extract and subjecting the concentrate to columnchromatography, for instance.

The above thiophenylation can be effected, for example by adding a basesuch as sodium hydroxide and sodium carbonate to a solution composed ofan optically active N-protected-β-chloroalanine and an amount of waterto give a starting material concentration of 5 to 30% (w/v), preferablyat 0 to 30° C., to thereby preferably adjust the pH to 9 to 11, andfurther adding thiophenol in an amount of 1 to 5 molar equivalents,preferably 1 to 3 molar equivalents, relative to the optically activeN-protected-β-chloroalanine, followed by stirring preferably at 30 to90° C., more preferably 40 to 70° C. The order of addition of thereagents is not always restricted to the one mentioned above. Forexample, the thiophenylation can also be effected by adding a base to anaqueous solution containing thiophenol and an optically activeN-protected-β-chloroalanine or by adding thiophenol and a basesimultaneously to an aqueous solution of an optically activeN-protected-β-chloroalanine.

In the production method of the present invention, by conducting,without isolating the intermediates, the three steps, namely the step oftreating an optically active serine or a salt of an optically activeserine with an acid with a chlorinating agent, the step of treating theresulting optically active β-chloroalanine with an amino-protectingagent and the step of reacting the resulting optically activeN-protected-β-chloroalanine with thiophenol under a basic condition, itis possible to obtained the corresponding optically activeN-protected-S-phenylcysteine derivative in a simple and efficientmanner. It is also possible to conduct, without isolating theintermediate, the two steps, namely the step of treating an opticallyactive β-chloroalanine with an amino-protecting agent and the step ofreacting the resulting optically active N-protected-β-chloroalanine withthiophenol under a basic condition.

The optically active N-protected-S-phenylcysteine obtained from thecorresponding optically active serine or a salt thereof by theproduction method of the present invention has an optical purity as highas 98% e.e. at the step prior to purification by crystallization, forinstance. Thus, according to the present invention, an optically activeN-protected-S-phenylcysteine having the same configuration as that ofthe substrate can be produced from the optically active serine or a saltthereof while substantially maintaining the optical purity thereofwithout accompanying substantial racemization.

The optically active N-protected-S-phenylcysteine, in particularN-carbobenzyloxy-S-phenyl-L-cysteine, is a compound very useful as anintermediate of HIV protease inhibitors (WO 9532185), for instance.

BEST MODES FOR CARRYING OUT THE INVENTION

The following examples illustrate the present invention in furtherdetail. They are, however, by no means limitative of the scope of thepresent invention.

EXAMPLE 1 Production of β-chloro-L-alanine Hydrochloride

L-Serine (5.0 g, 0.0476 mol) was added to 50 ml of 1,4-dioxane, andhydrogen chloride gas was introduced into the resulting solution withstirring at room temperature. On that occasion, the hydrogen chloride inthe solution amounted to 14.5 g (0.3977mol). To the solution was addedslowlyl 2.5 g (0.1051 mol) of thionyl chloride, and the reactor insidetemperature was then adjusted to 50° C. After about 6 hours of stirring,this solution was concentrated to about half the original volume. Theconcentrate was cooled to 0 to 10° C., and 50 ml of water was addedgradually so as to maintain this temperature. HPLC analysis of thissolution revealed the formation of 6.9 g (0.0431 mol) ofβ-chloro-L-alanine hydrochloride (yield: 91 mole %).

EXAMPLE 2 Production of β-chloro-L-alanine Hydrochloride

L-Serine (5.0 g, 0.0476 mol) was added to 50 ml of 1,4-dioxane, andhydrogen chloride gas was introduced into the resulting solution withstirring at room temperature. On that occasion, the hydrogen chloride inthe solution amounted to 11.2 g (0.3072 mol) . To the solution was addedslowly 6.2 g (0.0521 mol) of thionyl chloride, and the reactor insidetemperature was then adjusted to 45° C. After about 20 hours ofstirring, this solution was concentrated to about half the originalvolume. The concentrate (slurry) was filtered, the cake was washed with10 ml of 1,4-dioxane and the wet crystals were dried under reducedpressure (40° C., not higher than 10 mm Hg) to give dry crystals. HPLCanalysis of the crystals obtained revealed that the yield ofβ-chloro-L-alanine hydrochloride as pure substance was 7.2 g (0.0450mol).

The IR, ¹H-NMR and ¹³C-NMR data of the β-chloro-L-alanine hydrochlorideobtained were in complete agreement with those of the β-chloro-L-alaninehydrochloride purchased from Aldrich Chemical Co.

EXAMPLE 3 Production of β-chloro-L-alanine

Milk white crystals (purity 95.2% by weight, containing 3.6 g (0.0225mol) of β-chloro-L-alanine hydrochloride) obtained in the same manner asin Example 2 were added to 14 ml of water to give a slurry. This slurrywas completely dissolved by slowly adding about 2 g of concentratedhydrochloric acid. To the solution was added 0.1 g of 50% activatedcarbon, and the mixture was stirred at room temperature for about 10minutes. The activated carbon was filtered off under reduced pressureand washed with 1 ml of water. The filtrate and washings obtained werecooled to 0 to 10° C., and the pH was adjusted to 5.5 by graduallyadding a saturated aqueous solution of lithium hydroxide whilemaintaining that temperature, to give a slurry. Acetone (42 ml) wasgradually added to this slurry to thereby cause sufficient precipitationof crystals, the resulting mixture was cooled to −10 to 0° C. andmaintained at that temperature for about 1 hour. The precipitatecrystals were filtered off, the cake was washed with 14 ml of acetone,and the wet crystals obtained were dried under reduced pressure (40° C.,not higher than 10 mm Hg) to give 2.65 g of β-chloro-L-alanine as whitecrystals. HPLC analysis of these crystals revealed a purity of 99.9% byweight and a yield of pure β-chloro-L-alanine of 2.65 g (0.0214 mol).

The β-chloro-L-alanine obtained had an optical purity of not less than99.9% e.e. as determined by HPLC analysis under the conditions shownbelow.

<Analytical Conditions>

Column: Tosoh TSK-Gel Enantio L1 (4.6 mm×250 mm)

Mobile phase: 0.5 M CUSO₄ aq./acetonitrile=80/20

Column temperature: 40 ° C.

Detection wavelength: 254 nm

Flow rate: 1.0 ml/min

Retention time:

β-chloro-L-alanine 9.3 min

β-chloro-D-alanine 7.8 min

EXAMPLE 4 Production of β-chloro-L-alanine

L-Serine (30.0 g, 0.2855 mol) was added to 600 ml of 1,4-dioxane, andhydrogen chloride gas was introduced into the resulting solution withstirring at room temperature. On that occasion, the hydrogen chloride inthe solution amounted to 133.1 g (3.6508 mol) To the solution was addedslowly 40.8 g (0.3426 mol) of thionyl chloride, and the reactor insidetemperature was then adjusted to 40° C. After about 20 hours ofstirring, the liquid (slurry) was concentrated to about half theoriginal volume. The concentrate (slurry) was cooled to 0 to 10° C., and200 ml of water was added gradually so as to maintain that temperature,to thereby cause dissolution of the precipitate. The resulting solutionwas further concentrated until the weight became about 200 g, 3.0 g of50% activated carbon was then added, and the mixture was stirred at roomtemperature for about 10 minutes. The activated carbon was filtered offunder reduced pressure and washed with 10 ml of water. The filtrate andwashings obtained were combined and further concentrated to a weight ofabout 120 g. This concentrate was cooled to 0 to 10° C., and the pH wasadjusted to 5.5 by gradually adding a saturated aqueous solution oflithium hydroxide while maintaining that temperature, to give a slurry.To this slurry was gradually added 600 ml of acetone for effectingsufficient precipitation of crystals, and the slurry was then cooled to−10 to 0° C. and maintained at this temperature for about 1 hour. Theprecipitate crystals were filtered off under reduced pressure and thecake was washed with 100 ml of acetone. The wet crystals thus obtainedwere dried under reduced pressure (40° C., not higher than 10 mm Hg) togive 32.6 g of β-chloro-L-alanine as dry crystals. HPLC analysis of thecrystals revealed a purity of 99.8% by weight and a yield of pureβ-chloro-L-alanine of 32.5 g (0.2625 mol)

EXAMPLE 5 Production of β-chloro-D-alanine Hydrochloride

D-Serine (5.0 g, 0.0476 mol) was added to 50 ml of 1,4-dioxane, andhydrogen chloride gas was introduced into the resulting solution withstirring at room temperature. On that occasion, the hydrogen chloride inthe solution amounted to 11.5 g (0.3154 mol). To the solution was addedslowly 6.2 g (0.0521 mol) of thionyl chloride, and the reactor insidetemperature was then adjusted to 45° C. After about 20 hours ofstirring, this solution was concentrated to about half the originalvolume. The concentrate (slurry) was filtered, the cake was washed with10 ml of 1,4-dioxane and the wet crystals were dried under reducedpressure (40° C., not higher than 10 mm Hg) to give dry crystals. HPLCanalysis of the crystals obtained revealed that the yield ofβ-chloro-D-alanine hydrochloride as pure substance was 7.0,g (0.0438mol). The thus-obtained β-chloro-D-alanine hydrochloride had an opticalpurity of not less than 99.9% e.e. as determined by the same method asmentioned in Example 3.

EXAMPLE 6 Production of β-chloro-L-alanine Hydrochloride

L-Serine (5.0 g, 0.0476 mol) was added to 50 ml of each of the reactionsolvents specified in Table 1, and hydrogen chloride gas was introducedinto the resulting solution with stirring at room temperature untilsaturation with hydrogen chloride. To the solution was added slowly 12.5g (0.1051 mol) of thionyl chloride, and the reaction was effected underthe conditions shown in Table 1. The reaction mixture (slurry) wasconcentrated to about half the original volume. The concentrate wascooled to 0 to 10° C. and 50 ml of water was added slowly so as tomaintain this temperature. This solution was analyzed by HPLC and theyield as β-chloro-L-alanine hydrochloride was determined. The resultsthus obtained are shown in Table 1.

TABLE 1 Reaction Reaction Reaction Solvent temperature time Yield1,2-Dimethoxyethane 50° C. 10 hrs 97% Tetrahydrofuran 40° C. 30 hrs 92%Triethylene glycol Dimethylether 50° C. 10 hrs 93%

EXAMPLE 7 Production of (αS, βR)-α-amino-β-chlorobutyric AcidHydrochloride

L-Threonine (10.14 g, 0.0851 mol) was added to 100 ml of 1,4-dioxane,and hydrogen chloride gas was introduced into the resulting solutionwith stirring at room temperature. On that occasion, the hydrogenchloride in the solution amounted to 15.5 g (0.4251mol). To the solutionwas added slowly 12.2 g (0.1022 mol) of thionyl chloride, and thereactor inside temperature was then adjusted to 50° C. After about 10hours of stirring, this solution was concentrated to about half theoriginal volume. The concentrate (slurry) was filtered, the cake waswashed with 20 ml of 1,4-dioxane and the wet crystals were dried underreduced pressure (40° C., not higher than 10 mm Hg) to give drycrystals. HPLC analysis of the crystals obtained revealed that the yieldof (αS, βR)-α-amino-β-chlorobutyric acid hydrochloride as pure substancewas 12.2 g (0.0701 mol). [α]_(D) ²⁰+16.1° (c=1.0, water) (lit., [α]_(D)²⁰+17.8° (c=1.0, water) [CHIRALITY 9, 656-660 (1997)].

EXAMPLE 8 Production of (αS, βR)-α-amino-β-hydroxybutyric Acid

Milk white crystals [purity 94.9% by weight, containing 5.0 g (0.0287mol) of (αS, βR)-α-amino-β-hydroxybutyric acid hydrochloride] obtainedin the same manner as in Example 7 were added to 19 ml of water to givea slurry. This slurry was completely dissolved by slowly adding about2.8 g of concentrated hydrochloric acid. To the solution was added 0.1 gof 50% activated carbon, and the mixture was stirred at room temperaturefor about 10 minutes. The activated carbon was filtered off underreduced pressure and washed with 1 ml of water. The filtrate andwashings obtained were combined and cooled to 0 to 10° C., and the pHwas adjusted to 5.5 by gradually adding a saturated aqueous solution oflithium hydroxide while maintaining that temperature, to give a slurry.Acetone (58 ml) was gradually added to this slurry to thereby causesufficient precipitation of crystals, the resulting mixture was cooledto −10 to 0° C. and maintained at that temperature for about 1 hour. Theprecipitate crystals were filtered off, the cake was washed with 19 mlof acetone, and the wet crystals obtained were dried under reducedpressure (40° C., not higher than 10 mm Hg) to give 3.75 g of (αS,βR)-α-amino-β-chlorobutyric acid as white crystals. HPLC analysis ofthese crystals revealed a purity of 99.8% by weight and a yield of pure(αS, βR)-α-amino-β-chlorobutyric acid of 3.74 g (0.02272 mol). mp 176°C. (decomp.) (lit., mp 176° C. (decomp.) [Yakugaku Kenkyu, 33, 428-437(1961)].

The IR, ¹H-NMR and ¹³C-NMR data of the (αS, βR)-β-amino-β-chlorobutyricacid thus obtained as crystals were in complete agreement with those ofthe crystalline (αS, βR)-α-amino-β-chlorobutyric acid separatelysynthesized by the method mentioned below.

Reference Example 1 Alternative Synthesis of (αS,βR)-α-amino-β-chlorobutyric Acid

Using thionyl chloride and methanol, threonine was derivatized intothreonine methyl ester hydrochloride, which was then treated withthionyl chloride to give α-amino-β-chlorobutyric acid methyl esterhydrochloride. This was then converted to α-amino-β-chloropropionic acidhydrochloride by hydrolyzing with hydrochloric acid. Theα-amino-β-chloropropionic acid hydrochloride was crystallized andisolated by the same technique as mentioned in Example 8.

EXAMPLE 9 Production of β-chloro-L-alanine Hydrochloride

L-Serine hydrochloride (6.7 g, 0.0473 mol) was added to 50 ml of1,4-dioxane. To the solution was added slowly 6.8 g (0.0572 mol) ofthionyl chloride at room temperature, and the reactor inside temperaturewas then adjusted to 60° C. After about 3 hours of stirring, thissolution was concentrated to about half the original volume. Theconcentrate was cooled to 0 to 10° C., and 50ml of water was addedgradually so as to maintain this temperature. HPLC analysis of thissolution revealed the formation of 4.6 g (0.0287 mol) ofβ-chloro-L-alanine hydrochloride (yield 61 mole %).

Comparative Example 1

L-Serine (20.0 g, 0.1903 mol) was added to 49.8 g (0.4187 mol) ofthionyl chloride, and the mixture was warmed to 60° C. and stirred for 6hours. This solution was hydrolyzed and then analyzed by HPLC. Noβ-chloro-L-alanine peak was observed but peaks due to unreacted L-serineand various impurities were observed.

Comparative Example 2

L-Serine (15.0 g, 0.1427 mol) was added to 150 ml of toluene, andhydrogen chloride gas was blown into the resulting solution at roomtemperature until saturation. To this solution was added 37.4 g (0.3140mol) of thionyl chloride, and the mixture was then warmed to 80° C. andstirred for 20 hours, This solution was hydrolyzed and then analyzed byHPLC. Peaks of various impurities were observed and the peak ofβ-chloro-L-alanine corresponded only to a trace amount. (The abovereaction mixture contained a tar-like substance and had a deep blackcolor.)

Comparative Example 3

L-Serine (15.0 g, 0.1427 mol) was added to 150 ml of methylene chloride,and hydrogen chloride gas was blown into the resulting solution at roomtemperature until saturation. To this solution was added 37.4 g (0.3140mol) of thionyl chloride, and the mixture was then warmed to 40° C. andstirred for 16 hours, This solution was hydrolyzed and then analyzed byHPLC. Peaks of various impurities were observed and the peak ofβ-chloro-L-alanine corresponded only to a trace amount. (The abovereaction mixture contained a tar-like substance and had a deep blackcolor.)

EXAMPLE 10 Production of N-carbobenzyloxy-β-chloro-L-alanine

L-Serine hydrochloride (0.4 g, 2.84 mmol) and 0.029 g (0.28 mmol) oftriethylamine were suspended in 4 ml of diethylene glycol dimethylether. Thereto was added dropwise 0.67 g (5.68 mmol) of thionyl chlorideat room temperature in a nitrogen gas atmosphere. After 2 hours ofstirring at 60° C., 8 ml of water was added while maintaining thereaction mixture inside at 15° C. or below, and the whole mixture wasstirred at room temperature for 30 minutes. Further, 1.6 g of potassiumcarbonate was added to make the pH about 10 and, thereafter, 0.956 g(5.68 mmol) of benzyl chloroformate was added dropwise. After overnightstanding at room temperature, the reaction mixture was washed with ethylacetate, the aqueous layer obtained was cooled with ice and acidifiedwith 50% sulfuric acid and then extracted with ethyl acetate. Thesolvent was distilled off and the residue was purified by columnchromatography to give 0.3 g (1.16 mmol, 41%) ofN-carbobenzyloxy-β-chloro-L-alanine.

The N-carbobenzyloxy-β-chloro-L-cysteine obtained gave the following¹H-NMR and IR data.

¹H-NMR (400 MHz, CDCl₃) δ(ppm): 3.85-4.06 (m, 2H) , 4.80-4.82 (m, 1H),5.14 (s, 2H), 5.70 (d, J=7.8 Hz, 1H), 7.36 (s, 5H) IR (neat): 3034,1720, 1516, 1456, 1203, 1066, 855, 754, 698 (cm⁻¹)

EXAMPLE 11 Production of N-carbobenzyloxy-β-chloro-L-alanine

L-Serine hydrochloride (0.4 g, 2.84 mmol) and 0.029 g (0.28 mmol) oftriethylamine were suspended in 4 ml of 1,2-dimethoxyethane. Thereto wasadded dropwise 0. 67 g (5.68 mmol) of thionyl chloride at roomtemperature in a nitrogen gas atmosphere. After 2 hours of stirring at60° C., 8 ml of water was added while maintaining the reaction mixtureinside at 15° C. or below, and the whole mixture was stirred at roomtemperature for 30 minutes. Further, 1.6 g of potassium carbonate wasadded to make the pH about 10 and, thereafter, 0.956 g (5.68 mmol) ofbenzyl chloroformate was added dropwise. After overnight standing atroom temperature, the reaction mixture was cooled with ice and acidifiedwith 50% sulfuric acid. The solution obtained was analyzed by HPLC,which revealed the formation of N-carbobenzyloxy-β-chloro-L-alanine in ayield of 42% (1.18 mmol) . The analytical conditions were as shownbelow.

Analytical conditions(N-carbobenzyloxy-β-chloro-L-alanine/N-carbobenzyloxy-L-serine)

Column: YMC-Pack ODS-A A-303 (250 mm×4.6 mm)

Mobile phase: Phosphate buffer (pH=3.0):acetonitrile=60:40

Flow rate: 1.0 ml/min

Sample injection size: 20 μl

Sample solvent: acetonitrile

Retention time:

6.2 min (N-carbobenzyloxy-β-chloro-L-alanine)

3.9 min (N-carbobenzyloxy-L-serine)

EXAMPLE 12 Production of N-carbobenzyloxy-β-chloro-L-alanine

L-Serine hydrochloride (0.1 g, 0.71mmol) and7.2mg (0.07 mmol) oftriethylamine were suspended in a solvent composed of 1 ml ofacetonitrile and 0.1 ml of diethylene glycol dimethyl ether. Thereto wasadded dropwise 0.167 g (1.42 mmol) of thionyl chloride at roomtemperature in a nitrogen gas atmosphere. After 2 hours of stirring at60° C., 2 ml of water was added while maintaining the reaction mixtureinside at 15° C. or below, and the whole mixture was stirred at roomtemperature for 30 minutes. Further, 0.4 g of potassium carbonate wasadded to make the pH about 10 and, thereafter, 0.239 g (1.52 mmol) ofbenzyl chloroformate was added dropwise. After overnight standing atroom temperature, the reaction mixture was cooled with ice and acidifiedwith 50% sulfuric acid. The solution obtained was analyzed by HPLC bythe same procedure as mentioned in Example 11, which revealed theformation of N-carbobenzyloxy-β-chloro-L-alanine in a yield of 34% (0.24mmol).

EXAMPLE 13 Production of N-carbobenzyloxy-S-phenyl-L-cysteine

N-Carbobenzyloxy-β-chloro-L-alanine (0.108 g, 0.42 mmol) was dissolvedin 0.5 ml of water and, then, 0.097 g (0.92 mmol) of sodium carbonatewas added. Thereafter, 0.054 g (0.50 mmol) of thiophenol was addeddropwise at room temperature in a nitrogen gas atmosphere. After 2 hoursof stirring at 60° C., the reaction mixture was cooled with ice andacidified with 1 N hydrochloric acid, and then extracted with ethylacetate. The solvent was distilled off and the residue was purified bycolumn chromatography to give 0.112 g (0.34 mmol, 81%) ofN-carbobenzyloxy-S-phenyl-L-cysteine. The compound obtained had anoptical purity of not less than 98% e.e. The optical purity wasdetermined by HPLC. The analytical conditions are shown below.

Optical purity determination conditions(N-carbobenzyloxy-S-phenyl-L-cysteine/N-carbobenzyloxy-S-phenyl-D-cysteine)

Column: DAICEL CHIRALPAK AS (250 mm×4.6 mm)

Mobile phase: (hexane/tert-butyl methyl ether/tri-fluoroacetic acid=800/200/2):ethanol=85:15

Flow rate: 1.2 ml/min

Sample injection size: 10 μl

Temperature: 35° C.

Sample solvent: (hexane/tert-butyl methyl ether/tri-fluoroaceticacid=800/200/2):ethanol=80:20

Retention time:

4.5 min (N-carbobenzyloxy-S-phenyl-L-cysteine)

5.6 min (N-carbobenzyloxy-S-phenyl-D-cysteine)

The results of ¹H-NMR and IR analysis of theN-carbobenzyloxy-S-phenyl-L-cysteine obtained were as follows:

¹H-NMR (400 MHz, CDCl₃) δ(ppm): 3.41 (dd, J=5.1, 14.2 Hz, 2H), 4.61-4.63(m, 1H), 5.07 (s, 2H), 5.56 (d, J=7.3 Hz, 1H), 7.17-7.55 (m, 10H) IR(neat): 3036, 1686, 1532, 1281, 1059, 737 (cm⁻¹)

EXAMPLE 14 Production of N-carbobenzyloxy-S-phenyl-L-cysteine

N-Carbobenzyloxy-β-chloro-L-alanine (0.091 g, 0.35 mmol) was dissolvedin 0.45 ml of water and, then, 0.065 g (0.77 mmol) of sodium hydrogencarbonate was added. Thereafter, 0.046 g (0.42 mmol) of thiophenol wasadded dropwise at room temperature in a nitrogen gas atmosphere. After 2hours of stirring at 60° C., the reaction mixture was cooled with iceand acidified with 1 N hydrochloric acid, and then extracted with ethylacetate. The solvent was distilled off and the residue was purified bycolumn chromatography to give 0.097 g (0.29mmol, 84%) ofN-carbobenzyloxy-S-phenyl-L-cysteine. The product obtained had anoptical purity of not less than 98% e.e. as determined by HPLC analysisfollowing the same procedure as in Example 13.

EXAMPLE 15 Production of N-carbobenzyloxy-S-phenyl-L-cysteine

N-Carbobenzyloxy-β-chloro-L-alanine (0.137 g, 0.53 mmol) was dissolvedin 0.68 ml of water and, then, 0.58 ml of 2 N aqueous sodium hydroxidewas added. Thereafter, 0.069 g (0.63 mmol) of thiophenol was addeddropwise at room temperature in a nitrogen gas atmosphere. After 2 hoursof stirring at 60° C., the reaction mixture was cooled with ice andacidified with 1 N hydrochloric acid, and then extracted with ethylacetate. The solvent was distilled off and the residue was purified bycolumn chromatography to give 0.107 g (0.32 mmol, 61%) ofN-carbobenzyloxy-S-phenyl-L-cysteine. The product obtained had anoptical purity of not less than 98% e.e. as determined by HPLC analysisin the same manner as in Example 13.

EXAMPLE 16 Production of N-carbobenzyloxy-S-phenyl-L-cysteine

L-Serine hydrochloride (10.0 g, 70.6 mmol) and 0.073 g (7.1 mmol) oftriethylamine were dissolved in 100 ml of diethylene glycol dimethylether, and 16.8 g (141.2 mmol) of thionyl chloride was added dropwise atroom temperature in a nitrogen gas atmosphere. After 2 hours of stirringat 60° C., 200 ml of water was added while maintaining the reactionsystem at 15° C. or below, and the resulting mixture was stirred at roomtemperature for 30 minutes. Further, 50 g of potassium carbonate wasadded to make the pH about 10 and, then, 17.9 g (141.2 mmol) of benzylchloroformate was added dropwise. After overnight standing at roomtemperature, 10 g of potassium carbonate was again added to make the pHabout 10 and, then, 10.7 g (97.1 mmol) of thiophenol was added dropwiseat room temperature in a nitrogen gas atmosphere. After 2 hours ofstirring at 60° C., the reaction mixture was cooled with ice andacidified with 50% sulfuric acid, and extracted with ethyl acetate. Thesolvent was distilled off and the residue was purified by columnchromatography to give 8.7 g (26.2 mmol, 37%) ofN-carbobenzyloxy-S-phenyl-L-cysteine. The product obtained had anoptical purity of not less than 98% e.e. as determined by HPLC analysisin the same manner as in Example 13.

EXAMPLE 17 Production of N-carbobenzyloxy-S-phenyl-L-cysteine

β-Chloro-L-alanine hydrochloride (15.7 g, 98.1mmol) was added to 160 mlof water and dissolution was effected. The reactor inside was cooled to0 to 5° C. and the pH was adjusted to 10 by dropwise addition of about36 g of a 30% (by weight) aqueous solution of sodium hydroxide withvigorous stirring. While maintaining the inside temperature at 0 to 5°C., 20.5 g (120.0 mmol) of benzyl chloroformate was added dropwise over1 hour with vigorous stirring and then stirring was continued for 4hours, during which the pH of the reaction mixture was maintained at 9.5to 10.5 by dropwise addition of about 16 g of a 30% (by weight) aqueoussolution of sodium hydroxide. The reaction mixture obtained was assayedfor N-carbobenzyloxy-β-chloro-L-alanine by HPLC and the yield thereofwas found to be 25.1 g (97.5 mmol).

To the reaction mixture obtained was added dropwise 22.0 g (200.0 mmol)of thiophenol with vigorous stirring. During the dropping, the pH of thereaction mixture was maintained at 9.7to 10.3by dropwise addition ofabout 26g of a 30% (by weight) aqueous solution of sodium hydroxide. Ina nitrogen atmosphere, the inside temperature was raised to 50° C. andthe reaction was allowed to proceed for 3.5 hours, during which the pHof the reaction mixture was maintained at 9.7 to 10.3 by dropwiseaddition of about 1 g of a 30% (by weight) aqueous solution of sodiumhydroxide. To the reaction mixture obtained was gradually added dropwiseabout 20 g of concentrated hydrochloric acid over 3 hours with vigorousstirring to thereby adjust the slurry pH to 3. The resulting precipitatecrystals of N-carbobenzyloxy-S-phenyl-L-cysteine were filtered off underreduced pressure and sufficiently deprived of the liquid reaction mediumby washing with two 100-ml portions of water, to give wet crystals ofN-carbobenzyloxy-S-phenyl-L-cysteine [29.8 g (89.9 mmol) as pureN-carbobenzyloxy-S-phenyl-L-cysteine]. The optical purity of theN-carbobenzyloxy-S-phenyl-L-cysteine obtained was 99.9% e.e.

Comparative Example 4 Production of S-phenyl-L-cysteine

A 20% (by weight) aqueous solution of sodium carbonate e (2.23 g, 0.0042mol) was added to 0.97 g (0.0088 mol) of thiophenol, and the mixture wasstirred at room temperature for 0.5 hour. To this solution was added asolution composed of 1.08 g (0.0088 mol) of β-chloro-L-alanine andwater, and the reaction was allowed to proceed for 5 hours, during whichthe pH of the reaction mixture was maintained at 8 to 10 while adding5.14 g (0.0097 mol) of a 20% (by weight) aqueous solution of sodiumcarbonate. To the reaction mixture obtained were added 30 ml of toluene,20 ml of water and about 3 g of concentrated hydrochloric acid in anitrogen atmosphere to thereby adjust the pH to 0.5. The aqueous layerafter separation from the organic layer was washed with two 30-mlportions of toluene to remove the remaining portion of thiophenol, togive 34.3 g of an aqueous solution of S-phenyl-L-cysteine.

HPLC analysis of the aqueous solution obtained revealed that the yieldas pure S-phenyl-L-cysteine was 0.45 g (0.0023 mol, 26.0% yield) . Amarked extent of decomposition of β-chloro-L-alanine was observed.

INDUSTRIAL APPLICABILITY

The present invention, constituted as above, makes it possible toproduce β-halogeno-α-aminocarboxylic acids, which are useful as startingmaterials for the production of medicinals, as well as optically activeN-protected-S-phenylcysteines, which are useful as intermediates ofmedicinals, and intermediates thereof, in a simple, efficient andindustrially advantageous manner and on a commercial scale.

What is claimed is:
 1. A method of producing aβ-halogeno-α-aminocarboxylic acid or a salt thereof which compriseshalogenating the hydroxyl group of a β-hydroxy-α-aminocarboxylic acid,in which the basicity of the amino group in α-position is not masked bythe presence of a substituent on said amino group, or a salt thereofwith an acid by treating the same with a halogenating agent in a solventcontaining an ether type solvent.
 2. The method of producing accordingto claim 1, wherein the halogenating agent is a thionyl halide.
 3. Themethod of producing according to claim 2, wherein the thionyl halide isthionyl chloride.
 4. The method of producing according to claim 1,wherein the halogenating agent is used in an amount of 1 to 10 moles permole of the β-hydroxy-α-aminocarboxylic acid.
 5. The method of producingaccording to claim 1, wherein the ether type solvent is miscible withwater.
 6. The method of producing according to claim 5, wherein thewater-miscible ether type solvent comprises at least one speciesselected from the group consisting of 1,2-dimethoxyethane, 1,4-dioxane,tetrahydrofuran, diethylene glycol dimethyl ether, triethylene glycoldimethyl ether, tetraethylene glycol dimethyl ether and polyethyleneglycol dimethyl ether.
 7. The method of producing according to claim 1,wherein the treatment with the halogenating agent is carried out in thepresence of a hydrogen halide.
 8. The method of producing according toclaim 7, wherein the hydrogen halide is used in an amount exceeding 2.0molar equivalents relative to the β-hydroxy-α-aminocarboxylic acid. 9.The method of producing according to claim 7, wherein the treatment withthe halogenating agent is carried out in a state completely saturated oralmost saturated with the hydrogen halide gas.
 10. The method ofproducing according to claim 7, wherein the hydrogen halide is hydrogenchloride.
 11. The method of producing according to claim 1, wherein thetreatment with the chlorinating agent is carried out in the presence ofan amine or a salt thereof.
 12. The method of producing according toclaim 11, wherein the amine is a tertiary amine.
 13. The method ofproducing according to claim 1, wherein a coexisting hydrogen halideafter treatment with the halogenating agent is converted to a salt formby means of a basic lithium compound, and dissolved in a medium composedof a water-miscible organic solvent and water while theβ-halogeno-α-aminocarboxylic acid is caused to precipitate out in itsfree form.
 14. The method of producing according to claim 13, whereinthe water-miscible organic solvent comprises at least one speciesselected from the group consisting of 1,2-dimethoxyethane, 1,4-dioxane,tetrahydrofuran, diethylene glycol dimethyl ether, triethylene glycoldimethyl ether, tetraethylene glycol dimethyl ether, polyethylene glycoldimethyl ether, acetonitrile, methanol, ethanol, n-propanol,isopropanol, tert-butanol and acetone.
 15. The method of producingaccording to claim 14, wherein the water-miscible organic solvent isacetone.
 16. The method of producing according to claim 13, wherein thevolume ratio of the water-miscible organic solvent to water is not lessthan
 1. 17. The method of producing according to claim 13, wherein thefinal cooling temperature in the step of precipitation is not higherthan 10° C.
 18. The method of producing according to claim 1, whereinthe low-boiling components occurring in the reaction mixture are reducedor eliminated beforehand after treatment with the halogenating agent butbefore precipitation of the desired product.
 19. The method of producingaccording to claim 1, wherein, after treatment with the halogenatingagent, the β-halogeno-α-aminocarboxylic acid in hydrohalogenic acid saltform that has precipitated from the reaction mixture as such or afterconcentration thereof is recovered.
 20. The method of producingaccording to claim 1, wherein, after treatment with the halogenatingagent, the reaction solvent is replaced with water to give an aqueoussolution containing the β-halogeno-α-aminocarboxylic acid.
 21. Themethod of producing according to claim 1, wherein theβ-hydroxy-α-aminocarboxylic acid is serine, threonine, allothreonine orβ-phenylserine.
 22. The method of producing according to claim 21,wherein the β-hydroxy-α-aminocarboxylic acid is serine.
 23. The methodof producing according to claim 1, wherein theβ-hydroxy-α-aminocarboxylic acid is optically active.
 24. The method ofproducing according to claim 22, wherein the β-hydroxy-α-aminocarboxylicacid is L-serine.
 25. A method of producing an optically activeN-protected-β-chloroalanine of the general formula (2) or a saltthereof:

wherein R¹ represents an amino-protecting group and R⁰ represents ahydrogen atom or, taken together with R¹, an amino-protecting group,which comprises preparing an optically active β-chloroalanine of thefollowing formula (1) or a salt thereof:

from an optically active serine or a salt thereof with an acid by themethod of producing according to claim 1 and then treating the same withan amino-protecting agent.
 26. The method of producing according toclaim 25, wherein the halogenating agent is a thionyl halide.
 27. Themethod of producing according to claim 25, wherein the amino-protectingagent is benzyl chloroformate and the optically activeN-protected-β-chloroalanine is represented by the general formula (2) inwhich R⁰ is a hydrogen atom and R¹ is a carbobenzyloxy group.
 28. Themethod of producing according to claim 1, wherein the treatment with thehalogenating agent is carried out at a temperature of 40-80° C.